It is generally known that the standard types of stainless steels leave much to be desired with respect to galling resistance. In many commercial applications utilizing stainless steel, such as food processing apparatus, lubricants cannot be used to prevent galling of the steel surface. For food processing and other applications, several galling resistant stainless steel alloys were developed having superior galling resistance compared to conventional austenitic stainless steels. Two specialty galling resistant stainless steels, sold under the respective trademarks Nitronic 60.RTM. and Gall-Tough.RTM., have high threshold galling stress values (TGS), nominally 7 ksi (48 MPa) and 15 ksi (103 MPa), respectively. U.S. Pat. No. 4,039,356, Schumacher et al., describes the galling resistant austenitic stainless steel alloy sold under the trademark Nitronic 60.RTM.. That alloy consists essentially of, in weight percent (%):
______________________________________ % ______________________________________ C 0.001-0.25 Mn 6-16 Si 2-7 Cr 10-25 Ni 3-15 Mo 4.0 max. N 0.001-0.4 Cu 4.0 max. Fe Bal. ______________________________________
U.S. Pat. No. 4,814,140, Magee, Jr., assigned to Carpenter Technology Corp., assignee of the present application, describes a galling resistant austenitic stainless steel alloy sold under the trademark Gall-Tough.RTM.. That alloy consists essentially of, in weight percent:
______________________________________ % ______________________________________ C 0.25 max. Mn 2.0-7.0 Si 1.0-5.0 Cr 12-20 Ni 2.0-7.75 Mo 3.0 max. N 0.35 max. Cu 3.0 max. Fe Bal. ______________________________________
The austenitic stainless steel alloys described in Schumacher, et al. and Magee, Jr. provide galling resistance that is superior to the standard types of austenitic stainless steels. The alloys disclosed and claimed in Schumacher et al. and Magee, Jr. provide general corrosion resistance comparable to Type 304 stainless steel. That level of corrosion resistance is adequate for uses in many chloride-containing environments. However, some applications, such as valve components in the petrochemical industry, require galling resistance that is superior to conventional austenitic stainless steels and chloride corrosion resistance, especially pitting resistance, that is at least as good as that provided by AISI Type 316 stainless steel.
Type 316 stainless steel, an austenitic stainless steel, has very good chloride pitting resistance. However, Type 316 stainless steel has a nominal threshold galling stress less than 1 ksi (6.89 MPa). Known duplex stainless steels such as UNS S32950, UNS S31803, and UNS S32550 also provide good pitting resistance, but each has a threshold galling stress less than 1 ksi (6.89 MPa). Thus, neither Type 316 nor the known duplex stainless steels have the combination of galling and pitting resistance necessary for petrochemical applications.
It would be highly desirable to have a stainless steel alloy which provides a superior combination of galling resistance and pitting resistance compared to known stainless steels such as Type 316, Gall-Tough.RTM., or Nitronic 60.RTM..